The present invention relates to a process for treating an aqueous slurry of pigment whereby the process and operation for manufacturing the pigment is simplified with industrial advantages imparted thereto.
Generally, a process for manufacturing a pigment comprises a chemical reaction step for preparing a pigment and precipitating the pigment in a medium, and a treatment step of the precipitated pigment including filtration, washing, drying, grinding and packing.
As the pigment there is mostly employed a pigment having a particle size of nearly 0.01 to 1 .mu.m. In an organic pigment, a pigment having a particle size of about 0.2 .mu.m is suitably employed for attaining a brilliant and clear color.
However, an aqueous slurry containing such fine pigment particles cannot be agitated with ease because the slurry is pasty and is mostly thixotropic. In addition, when the slurry is vigorously agitated, air foams formed in the slurry are hardly removed, and then the slurry becomes creamy.
In order to avoid the above industrial defects, production of a pigment is conducted in general by using a dilute aqueous slurry. For instance, the pigment slurry prepared by the reaction step has a pigment concentration of not more than 10% (% by weight, hereinafter the same), mostly 1 to 5%.
In the pigment product there remains a salt resulting from the mother liquor contained in the pigment cake which is separated from the aqueous slurry. In case of reducing the content of the salt, which is required in a pigment product for a printing ink, or the like, there is usually employed a process in which a pigment is prepared under the condition of a low concentration, and then the pigment prepared is washed with water by repeating dilution and decantation. The treatment in which filtration is carried out together with the repeated dilution and decantation is only employed in an unavoidable case where the washing efficiency of the pigment cake is remarkably low with any filter.
The water content of the filter cake thus obtained is 70 to 80% of that of a wet cake obtained by using a filter press, and is 80 to 90% of that of a wet cake obtained by using a rotary vacuum filter. It is very difficult to lower the water content further. Though use of a centrifugal filter can reduce the water content of a filter cake more than the use of the filter press, when the centrifugal filter is used for filtering a pigment slurry containing pigments of about 0.2 .mu.m in particle size, there is a defect in that the filter was choked with the pigment particles to make the operation difficult.
In addition, there is an industrial disadvantage in that energy consumption required for drying the filter cake is large because it is necessary to evaporate water which weight is 3 to 5 times the weight of the solid content.
Furthermore, the pigment of such fine primary particles tends to form a hard mass. Particularly, a pigment containing a hydrophilic group has the greater tendency to do so. Grinding operation of the hard mass of the pigment is very troublesome, and additionally often has undesirable effects on the colour tone of the pigment product. The reason is assumed that the mechanical grinding cannot divide the secondary particles which are formed by agglomeration in the drying step from the original primary particles. As mentioned above, there are many problems in the conventional grinding and packing steps such as degradation in quality of the pigment product resulting from shape and size of such fine particles, and contamination of dirt occurred in handling.
An object of the present invention is to provide an industrially advantageous process for treating an aqueous slurry of pigment whereby the process for manufacturing the pigment is simplified and the above-mentioned problems are eliminated.